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Simple VLAN question ...

Posted on 2003-10-29
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Last Modified: 2010-04-11
hi,

I now have a flat L2 network which I plan to divide up by assigning VLAN 2 to finance, VLAN3 to HR, VLAN4 to Engineering... however, they all need to access a few servers (eg email) that should be accessible to all the VLANs - thus the server port would have VLAN2,3,4 configured.

Note that no L3 devices is required because the IP addressing remains in one subnet (eg. 10.10.10.x) for the whole company. This is to control broadcast domains, and improve security. My aim is improve security with the least disruption. Putting in a L3 device (such as a router-on-a-stick or L3 switch) would require to change IP addressing - which would probably be a major task by itself - looking at the number of clients to be changed.
 
Is this approach workable and recommended? What are your comments / suggestions / advise.. Thanks!!!

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Question by:Haho
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by:Haho
ID: 9647138
Fyi, I understand that without a L3 device, VLAN 2 users will not be able to talk to VLAN 3 users. That's why shared resources such as servers, gateways, printers, etc will belong to multiple VLANs.
 
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by:NicBrey
ID: 9648682
Answer is no.  It is not possible to have an access port on a switch be part of more than one VLAN. Only trunk ports can do that.
The whole idea of VLANs is to break up the broadcast domain on lan segments. Unfortunately, for your idea to work, you would have to get a layer 3 switch or router and redesign your addressing scheme.

Another option is to bridge the VLANs. Do that by putting a cross-over cable into a port on VLAN1 and VLAN2.  If you do this, you might as well not have VLANs though.  The only true solution is to have the VLANs in different networks/subnets and to route or layer3 switch between them.
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by:Haho
ID: 9649577
Hi Nic,

I think the answer is "yes" ? - it is called "multi-VLAN port"
from the net, I found:
>>>
To assign a multi-VLAN port to a vlan, from interface configuration mode issue the command "switchport mode multi" followed by "switchport multi vlan v1 v2 ... vn" where "v1 v2 ... vn" is a list of vlan numbers. For example, to add port 13 to vlans 2, 3, and 6:

     switch#interface fastEthernet 0/13
     switch(config-if)# switchport mode multi
     switch(config-if)# switchport multi vlan 2 3 6

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NicBrey earned 250 total points
ID: 9649924
Hi Haho,

Yes, that slipped my mind a bit....
The multi-VLAN port you talk about is like the trunk port that I mentioned earlier.  Ideally you will have all VLANs in different networks and the multi-VLAN port connects to a router on a stick like a trunk does.

VLAN assignments and Mutli-VLAN ports and trunking.
http://www.cisco.com/en/US/products/hw/switches/ps605/products_configuration_guide_chapter09186a008007d7dd.html#xtocid119665

Overlapping VLANs.
http://www.cisco.com/en/US/products/hw/switches/ps607/products_installation_and_configuration_guide_chapter09186a008007d32c.html#xtocid228683

Since you will have all PC's on one subnet, it will work because the link is not actually a trunked link - just carrying traffic for more than one VLAN. So yes, go ahead and try it...
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